The present invention relates generally to integrated circuit (IC) fabrication equipment. More particularly, the present invention relates to an extreme ultraviolet (EUV) mask or reticle used in IC fabrication.
Integrated circuit (IC) fabrication often utilizes a mask or reticle to form an image or pattern on one or more layers comprising a semiconductor wafer. Radiation is provided through or reflected off the mask or reticle to form the image on the semiconductor wafer. The wafer is correspondingly positioned to receive the radiation transmitted through or reflected off the mask or reticle. The radiation can be light at a wavelength in the ultraviolet (UV), vacuum ultraviolet (VUV), deep ultraviolet (DUV), or extreme ultraviolet (EUV) range. The radiation can also be a particle beam such as an x-ray beam, an electron beam, etc.
Typically, the image on the mask or reticle is projected and patterned onto a layer of photoresist material disposed on the wafer. The areas of the photoresist material upon which radiation is incident undergo a photochemical change to become suitably removable or fixed in a subsequent development process. In turn, the patterned photoresist material is used to define doping regions, deposition regions, etching regions, and/or other structures comprising the IC.
Radiation or lithographic wavelengths in the EUV range are being considered for next-generation lithography. EUV lithography utilizes radiation at a wavelength in the range of approximately 5 to 70 nanometer (nm) (e.g., 13.4 nm). EUV lithography requires, among others, the use of a mask or reticle specifically configured for lithographically patterning at EUV wavelengths, and is commonly referred to as an EUV mask or reticle. Unlike conventional masks or reticles which project a pattern onto the photoresist material by selective transmission of the exposure radiation, EUV masks or reticles project the pattern onto the photoresist material by selective reflection of the exposure radiation. For this reason, EUV masks or reticles are also referred to as reflective or reflective medium masks or reticles. The radiation at an EUV wavelength is reflected off the EUV mask or reticle in accordance with a pattern provided thereon and this reflected radiation is configured by components such as an EUV lens assembly before being projected onto the photoresist material.
In FIG. 1, a cross-sectional view of a portion of an EUV mask or reticle 10 is shown. EUV mask 10 includes a substrate 12, a multilayer 14, a barrier or buffer layer 16, an absorptive layer 18, and a feature 20. Multilayer 14 is disposed above substrate 12, barrier layer 16 is disposed over multilayer 14, and absorptive layer 18 is disposed over barrier layer 16.
Substrate 12 is typically a low-thermal expansion material (LTEM), such as glass. Multilayer 14 is configured to reflect radiation in the EUV wavelength range. Multilayer 14 is comprised of a plurality of films, such as 20 to 40 molybdenum-beryllium (Moxe2x80x94Be) film pairs or molybdenum-silicon (Moxe2x80x94Si) film pairs. The material comprising barrier layer 16 is selected to have different etch characteristics than at least absorptive layer 18. For example, barrier layer 16 can be a silicon dioxide material.
Absorptive layer 18 includes a metallic material, such as chromium. The portion of absorptive layer 18 shown in FIG. 1 comprises feature 20. Feature 20 forms a part of the pattern or image for EUV mask 10 and may also be used to etch layer 16.
Ideally, EUV mask 10 is configured to selectively reflect or absorb radiation at an EUV wavelength incident thereon to project a pattern onto the wafer. In particular, the exposed areas of multilayer 14, such as, areas 22, reflect radiation while the remaining areas of multilayer 14, such as, area 24, are covered by absorptive layer 18 to be non-reflective. Presently, however, all of the materials considered for absorptive layer 18 cause a certain amount of reflection. Moreover, as shown in FIG. 2, when a reflected ray 26 (from the top surface of absorptive layer 18) and a reflected ray 28 (from the bottom surface of absorptive layer 18) are in phase with each other, they are additive such that not only is absorptive layer 18 reflective, rather than preventing reflections, but reflections therefrom may be quite strong in intensity. In extreme cases, absorptive layer 18 can act as reflective areas of EUV mask 10 (similar to areas 22 of multilayer 14) rather than being absorbing or non-reflective areas. Hence, the pattern projected onto the wafer (i.e., the pattern defined by the reflections from EUV mask 10) may not be the desired pattern (i.e., the pattern as defined by the exposed areas of multilayer 14 and absorptive layer 18). Alternatively, the pattern projected onto the wafer may suffer from low image contrast.
Thus, there is a need for an EUV mask configured to provide improved image contrast. There is a further need for an EUV mask having an absorptive layer of conventional material that provides reduced reflectance.
One exemplary embodiment relates to a reflective mask or reticle. The mask or reticle includes an absorptive layer configured to reduce a reflection of a lithographic radiation having a wavelength shorter than in a deep ultraviolet (DUV) range by destructive interference.
Another exemplary embodiment relates to a reflective mask or reticle. The mask or reticle includes an absorptive layer. The mask or reticle further includes an anti-reflective coating (ARC) layer disposed over the absorptive layer. The ARC layer is configured to reduce a reflection of a lithographic radiation having a wavelength shorter than in a deep ultraviolet (DUV) range from the absorptive layer by destructive interference.
Still another exemplary embodiment relates to a method for reducing an absorptive layer reflection from an absorptive layer of a reflective mask or reticle. The reduction of the absorptive layer reflection is during lithography at a wavelength shorter than in a deep ultraviolet (DUV) range. The method includes generating an another reflection. The method further includes canceling the absorptive layer reflection using the another reflection.